一个C#编写的目录树源文件，可以自由展开的,望各位高手提供源代码

using System;
using System.Drawing;
using System.Collections;
using System.ComponentModel;
using System.Windows.Forms;
using System.IO;

namespace folderSelect
{
 /// <summary>
 /// Form1 的摘要说明。
 /// </summary>
 public class Form1 : System.Windows.Forms.Form
 {
  private static string driveLetters = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
  private System.Windows.Forms.TreeView treeView1;
  /// <summary>
  /// 必需的设计器变量。
  /// </summary>
  private System.ComponentModel.Container components = null;

  public Form1()
  {
   //
   // Windows 窗体设计器支持所必需的
   //
   InitializeComponent();
   this.fixtree();

   //
   // TODO: 在 InitializeComponent 调用后添加任何构造函数代码
   //
  }

  /// <summary>
  /// 清理所有正在使用的资源。
  /// </summary>
  protected override void Dispose( bool disposing )
  {
   if( disposing )
   {
    if(components != null)
    {
     components.Dispose();
    }
   }
   base.Dispose( disposing );
  }

  #region Windows Form Designer generated code
  /// <summary>
  /// 设计器支持所需的方法 - 不要使用代码编辑器修改
  /// 此方法的内容。
  /// </summary>
  private void InitializeComponent()
  {
   this.treeView1 = new System.Windows.Forms.TreeView();
   this.SuspendLayout();
   //
   // treeView1
   //
   this.treeView1.ImageIndex = -1;
   this.treeView1.Location = new System.Drawing.Point(0, 8);
   this.treeView1.Name = "treeView1";
   this.treeView1.SelectedImageIndex = -1;
   this.treeView1.Size = new System.Drawing.Size(296, 264);
   this.treeView1.TabIndex = 0;
   this.treeView1.BeforeExpand += new System.Windows.Forms.TreeViewCancelEventHandler(this.treeView1_BeforeExpand);
   //
   // Form1
   //
   this.AutoScaleBaseSize = new System.Drawing.Size(6, 14);
   this.ClientSize = new System.Drawing.Size(292, 273);
   this.Controls.AddRange(new System.Windows.Forms.Control[] {
                    this.treeView1});
   this.Name = "Form1";
   this.Text = "Form1";
   this.ResumeLayout(false);

  }
  #endregion

  /// <summary>
  /// 应用程序的主入口点。
  /// </summary>
  [STAThread]
  static void Main()
  {
   Application.Run(new Form1());
  }

  private void fixtree()
  {
   foreach( char c in driveLetters )
   {
    string sCurPath = c + ":\\";
    try
    {
     
     DirectoryInfo directory = new DirectoryInfo(sCurPath);
     if ( directory.Exists == true )
     {
      TreeNode newNode = new TreeNode(directory.FullName);
      treeView1.Nodes.Add(newNode); // add the new node to the root level.
      getSubDirs(newNode);   // scan for any sub folders on this drive.
     }
    }
     
    catch( Exception doh)
    {
     Console.WriteLine(doh.Message);
    }
   }
  }
  private void getSubDirs( TreeNode parent )
  {
   
   DirectoryInfo directory;
   try
   {
    // if we have not scanned this folder before
    
    if ( parent.Nodes.Count == 0 )
    {
     directory = new DirectoryInfo(parent.FullPath);
     foreach( DirectoryInfo dir in directory.GetDirectories())
     {
      TreeNode newNode = new TreeNode(dir.Name);
      parent.Nodes.Add(newNode);
     }
    }

   }
   catch( Exception doh )
   {
    Console.WriteLine(doh.Message);
   }
  }

  private void treeView1_BeforeExpand(object sender, System.Windows.Forms.TreeViewCancelEventArgs e)
  {
   foreach(TreeNode node in e.Node.Nodes)
   {
    this.getSubDirs(node);   
   }
   
  }
  
 }
}